Removal of acidic gases from gas streams

ABSTRACT

THIS IS A CYCLIC PROCESS FOR THE REMOVAL AND RECOVERY OF SULFUR DIOXIDE FROM WASTE STACK GASES TO LESSEN ATMOSPHERIC POLLUTION. THE PROCESS INVOLVES: (A) ELECTROLYTICALLY CONVERTING A SALT SOLUTION INTO AN ACID AND BASE; (B) EMPLOYING THE BASE TO ABSORB THE SULFUR DIOXIDE FROM THE WASTE GAS; (C) NEUTRALIZING THE RESULTING SPENT BASE WITH THE ELECTROLYTICALLY PRODUCED ACID TO REFORM THE ORIGINAL SALT SOLUTION AND TO REALEASE THE ABSORBED SULFUR DIOXIDE GAS, AND   (D) RECYCLING THE SALT SOLUTION TO THE ELECTROLYTIC CELL AND RECOVERING SULFUR DIOXIDE GAS.

1971i VW.;AZQ.QMORAE T 5 REMOVAL' O ACIDIC GASES FROM GAS STREAMS p iinal siled'march zz; 1967: IS-Sheets-Sheet 1 INVENTORS WAYNE A. MC RAEDANIEL L. BROWN STUART A. MO GRIFF ATTORNEY REMOVAL QR; AQIIDIC GASESFROM GAS STREAMS ori mal "riieqfmarmzz, 'SSheets-Sheet z II'IIIIIIIIII(III INVENTORS WAYNE A; MC RAE DANIEL L. BROWN STUART A. MC GRIFF 'MQAAQLJ ATTORNFY WAItR J flz; 1971 w.f.,= \"fyRgg ETAL, I 3,554,895

' "REMOVAL OF-IYACIDIC GASES FROM GAS STREAMS Origixial Filed March 22;1967 3 Sheets-Sheet s N I 1 I 8 n /MM? MW ATTORNEY United States Patent3,554,895 REMOVAL OF ACIDIC GASES FROM GAS STREAMS Wayne A. McRae,Lexington, and Daniel L. Brown, Wayland, Mass., and Stuart G. McGriff,Alexandria, Va.,

assignors to Ionics, Incorporated, Watertown, Mass. Original applicationMar. 22, 1967, Ser. No. 625,149, new

Patent No. 3,475,122, dated Apr. 18, 1969. Divided and this applicationMay 19, 1969, Ser. No. 837,004

Int. Cl. B01d 13/02 US. Cl. 204301 8 Claims ABSTRACT OF THE DISCLOSUREThis is a cyclic process for the removal and recovery of sulfur dioxidefrom waste stack gases to lessen atmospheric pollution. The processinvolves:

(a) electrolytically converting a salt solution into an acid and base;

(b) employing the base to absorb the sulfur dioxide from the waste gas;

(0) neutralizing the resulting spent base with the electrolyticallyproduced acid to reform the original salt solution and to release theabsorbed sulfur dioxide gas, and

(d) recycling the salt solution to the electrolytic cell and recoveringsulfur dioxide gas.

This application is a division of Ser. No. 625,149 filed Mar. 22, 1967,now US. Pat. 3,475,122.

This invention is concerned with a method for the recovery ofsalt-forming gases from gaseous streams and in particular to the removaland recovery of acidic gases from waste gases. It relates specificallyto the selective recovery of sulfur dioxide from a more or less arid gasstream which may also contain other acidic gases.

Sulfur dioxide (S0 is a component of many gaseous eflluents such as fluegases (boiler and furnace exhausts), smelter gases, off-gases fromchemical and petroleum processes, and stack gases produced from burningsulfur-containing hydrocarbon fuels, such as oil, sour natural gas andcoal. Pollution of the atmosphere by sulfur dioxide has been a problemfor many years due to its noxious effect on human, animal and plantlife, and on metals and other materials. Many methods have been proposedfor removing sulfur dioxide from gaseous eflluents. One such process isbased on the oxidation of S0 to S0 employing a catalyst such as vanadiumpentoxide. The S0 is subsequently recovered as sulfuric acid. For itsapplication to flue gas from the combustion of coal or hydrocarbons, theprocess requires the use of a high temperature electrostaticprecipitator to remove fly ash so as not to plug the catalyst bed. Thehigh water content of flue gas generally results in the formation ofdilute acid, for example, 70% sulfuric acid as a final product,requiring costly corrosionresistant equipment. The product of 70 percentacid is difficult to market without expensive concentration to 96 to 98percent acid. Another process utilizes alkalized alumina. An alkalimetal oxide is supported on spheres of alumina. The S0 from the flue gasis absorbed on the spheres in free fall. The spent absorbent isregenerated by heating to a high temperature, for example 1,200 F. witha reducing gas. The disadvantage of this process is that the absorbentloses activity and degrades during repeated regeneration cycles. A thridprocess utilizes activated charcoal. Sulfur dioxide in the flue gas isoxidized to S0 and absorbed with endogenous water on the charcoal toform sulfuric acid. The charcoal with its adsorbed acid is regeneratedby heating to about 700 F. This process suffers from the degradation ofthe charcoal at the regeneration temperatures and the corrosion of thestruc- 3,554,895 Patented Jan. 12, 1971 tural materials in theregeneration and adsorber apparatus. The prior art processes areexpensive and inefficient, especially when the S0 concentration in thegas is less than 1 percent. Other objections to certain processes arethe lack of practical ways, first of ultimately disposing of the S0pollutant or of recovering it in a usable form and second, ofregenerating the absorbent material for re-use. The dry processesemployed for S0 elimination have serious disadvantages in that there isslow penetration of the S0 into the solid adsorbent resulting in thereaction of only a small portion of the absorbent material. Further, itis diflicult to regenerate the absorbents due to fouling of the surfaceof the solid by dust in the gas. Processes employing liquid-scrubbingsolutions, such as ammonia or various amines, although technicallyfeasible, have not been adopted because of the high cost involved in theinitial chemical and its subsequent regeneration.

It is therefore the object of this present invention to provide animproved cyclic electrolytic process for the liquid absorption ofsalt-forming gases, especially S0 from gas streams in which they occurand their subsequent release and recovery by neutralization of theabsorbing material.

Another object is to provide a process that removes S0 at high percentefliciencies simultaneously with removal of heat, dust and/or fly ashparticles contained in the gas.

Another object is to recover the salt-forming gases in a commerciallyuseful and salable form.

Another object is to provide substantially complete and readyregeneration of the absorbent material without the use of hightemperature or outside chemicals.

A further object is to provide a process having low equipment cost,simplicity and dependability.

Various other objects and advantages will appear from the followingdisclosure and the novel features will be particularly pointed outhereafter in connection with the appended claims.

In general, the invention as disclosed herein comprises a continuousself-regenerating liquid-phase absorption system employing a novelcombination of four basic steps for controlled stack gas purification.The first step involves the electrolytic conversion of an ammonium oralkali metal sulfate salt into its corresponding acid and base. Thesecond step involves the absorption of sulfur dioxide from the gasstream into the aqueous basic solution (caustic) to form predominatelybisulfite solution. The third step is directed to neutralizing the spentcaustic containing the bisulfite with the electrolyticall produced acidto reform the original alkali metal salt and recover the sulfur dioxide.In the fourth step, this salt solution is reclarified and recycled withor without concentration or dilution as feed to the electrolytic celland again electrolytically converted to the acid and base. During theneutralization step, the S0 is desorbed from the spent caustic and iscollected as a valuable chemical which, for example, may be ultimatelyconverted to sulfuric acid by well known processes. Where the stack gasalso contains CO the S0 being a stronger acidic gas will be removed inpreference to the CO Since equivalent amounts of acid and base areinherently generated by the cell, there is no chemical disposal problem.Water may be required to make up for evaporation and for losses due tothe electrolytic decomposition of water into oxygen and hydrogen gases.

The principles and features of the invention are readily understood whentaken in connection with the accompanying drawings by considering thebasic steps for practicing the same. It is understood that details maybe modified or varied without departure from the principles of theinvention. The drawings are schematic diagrams of apparatus illustratinggenerally the flow of materials and treatment thereof. For the purposeof simplicity the vari- 3 ous valves, fiowmeters, pressure gauges,switches, pumps, etc. which one skilled in the art might employ in thepractice of the present invention are not all fully illustrated in thedrawings.

The process for carrying out the invention will be described by way ofexample by reference to the apparatus shown schematically in FIG. 1 andin particular to the employment of potassium sulfate as the electrolytefeed solution to the electrolytic cell. In the practice of theinvention, a feed solution of potassium sulfate is passed from line 40by pump 41 to the electrolytic cell 1 and by means of a source of directcurrent passed to the cell through leads 50 and 51 (source not shown)the potassium sulfate is split, resulting in the formation of sulfuricacid and potassium hydroxide. The electrolytic cell is preferably of thetype having three compartments, wherein the partition between the anodecompartment 2 and the center compartment 3 is a diaphragm 4 ofcontrolled porosity. Between the cathode compartment 5 and the centercompartment 3 there is preferably a cation-permselective membrane 6. Thecation membrane prevents bulk mixing of the center and cathodecompartment solutions. If desired, the cation permselective membrane canbe replaced with a second controlled porosity diaphragm. Thenon-permselective diaphragm 4 is of a design that will allow passage ofelectrolyte solution therethrough but restrict flow of gaseous anodicproducts, such as oxygen. The diaphragm is preferably of such suitableacid-resistant microporous materials as, for example, rubber, ceramic,polyethylene, canvas, asbestos, Teflon and other synthetic fabrics, aswill be more fully discussed below.

The cation permselective membrane is commonly of the type consisting ofcation exchange substance prepared in the form of thin sheets; andmembranes being substantially hydraulically impermeable to water and toions carrying a negative charge but permeable to ions carrying apositive charge. The art contains many examples of cation exchangematerials which can be formed into cation permselective membranes.

The anode compartment is provided with an acid resistant anode 7 forexample, lead, chilex, a tungsten bronze, platinum or platinum-coatedelectrolytic valve metals), as will be more fully set forth below, anoutlet 8 for the anolyte efiluent product and outlet 9 for gaseousanodic products such as oxygen. The center compartment contains an inlet10 through which the electrolyte feed solution is introduced.

The cathode compartment 5 defined from the center compartment 3 by thecation membrane 6 is provided with a caustic resistant cathode 11 suchas copper, lead, nickel, iron or steel and an inlet 12 through whichelectrolyte or water is passed. Outlet 13 serves to withdraw the causticcatholyte effluent product, and outlet 14 removes gaseous cathodicproducts such as hydrogen. The diaphragm, membrane and electrodecomponents may be separated from each other by thin, gasketed spacers(not shown) which form the fluid-containing compartments of the cell.

In operation, a solution of an electrolyte, for example, sodium sulfateor potassium sulfate is introduced under pressure into the centercompartment through inlet 10 at a rate and pressure which in its passagethrough the porous diaphragm (as shown by the arrow) is sufficient tosubstantially prevent the hydrogen ions formed at the anode frommigrating to the cathode in competition with the passage of alkali metalcations into said cathode. Simultaneously, electrolyte (or preferablywater, as will be more fully discussed) is passed into the cathodecompartment via inlet 12 at a rate depending on the concentration ofcaustic desired in the catholyte effluent product and on the impresseddirect current employed. Under the influence of an impressed directelectric current, the cationic constituents of the electrolytic solutionin the center compartment, for example, potassium ions, pass through thecation permselective membrane into the cathode compartment. Thecombination of such alkali metal ion with hydroxyl ions produced at thecathode by the electrolysis of water forms the corresponding hydroxide,for example, potassium hydroxide. This catholyte product is withdrawnthrough outlet 13 in a concentration dependent upon the current employedand the rate of liquid flow into the cathode compartment. Theelectrolytic solution in the center compartment 3 now having beenpartially depleted of its positive ions (e.g. potassium) passes throughthe porous diaphragm into the anode compartment where combination of itsfree anionic groups (e.g. with hydrogen ions produced by theelectrolysis of water at the anode forms the corresponding acid, forexample, sulfuric acid or potassium bisulfate. The anolyte is withdrawnthrough Outlet 8 as a mixture of the original unreacted salt and itscorresponding acid. In the case, for example, wherein 50% of thepotassium ions of a potassium sulfate solution are effectivelytransferred to the cathode compartment, the resultant anolyte eflluentproduct will be a equimolar solution of sulfuric acid and potassiumsulfate.

The processes may be more clearly understood with reference to thefollowing series of equations wherein the electrlytic salt employed ispotassium sulfate:

( 1) At the cation exchange membrane 6:

Center Ki (excess) [Compartment Anode K+ (excess( [CompartmentApplication of these processes toward the electrolysis of inorganicsalts, for example, potassium sulfate, sodium sulfate, ammonium sulfate,sodium nitrate and potassium nitrate, whose corresponding acids arestrongly acidic, results in the production of an anolyte eflluentproduct comprising a mixture of such acid with the original inorganicsalt, the ratio between the two constituents being determined by therate at which the electrolytic feed solution is introduced into thecenter compartment and the impressed current to the cell. The flow rateof the electrolytic solution may be regulated so that the acid andorginal salt content of the anolyte product is of any desired value. Forexample, in the case of potassium sulfate, the flow of an aqueoussolution of the same into the center compartment may be regulated sothat the anolyte efiluent product is equivalent in acid and potassiumsulfate. The eflluent caustic product from the cathode compartmentcontaining any unconverted salt is passed via line .15 into the optionalcaustic hold-up tank 16, subsequently withdrawn by pumping means 17 andpassed through line 18 into the top of the absorber or scrubber tower 19which may be, for example, a conventional counter-current packed toweror a spray tower. Simultaneously with the flow of caustic, a gas streamcontaining S0 and/or other salt-forming gases is introduced into thebottom of the tower at 20 through inlet gas line 21 by means of optionalgas inlet pump 22 or other pumping means. The waste gas is preferablypassed upwardly in counter-current flow to the caustic solution whichenters the top of the tower at 23. The caustic may, for example, besprayed downwardly therein in the form of small droplets by a series ofnozzles 24. The tower may instead contain bubble trays to bring aboutintimate contact of the gases and the caustic scrubbing solution. Thetower may alternately be packed with ceramic or plastic materials havingthe shape of rings, saddles, tellerettes, etc. Packed column absorbersare best operated counter-currently so as to allow contacting the lesscontaminated gases with the most avid liquid-absorbing material. Thedescending caustic will absorb acidic substances such as S S0 CO and N0and small particulate matter, such as fly ash, and then collect in thebottom of the tower at 25. The tower can be designed so that the causticsolution makes a single downward pass through the absorber. To improvethe performance of the scrubber, the caustic can be continuouslyrecirculated therethrough by pumping means 26, a portion of the liquidbeing removed from the bottom of the tower and returned to inlet line 18by means of return or recycle conduit 27. This recirculation providescontinuous contact with the upwardly moving gas. The depleted gas, afterpassing upwardly through the tower, is removed from the tower at gasexit line 28. Where a single pass of the laden gas is not sufiicient toremove the desired percentage of sulfur dioxide, part of the gas may berecycled by a pump 29 back to the bottom of the tower for furtherscrubbing by way of return conduit 30. Preferably, at least 80% S0removal should be accomplished in a single pass or by recycling.

The spent or exhausted caustic solution is continuously bled from theabsorber, carried away from the tower by outlet 31 and passed into theneutralizer, desorber, stripper or regeneration tank 32 by pumping means43. Caustic from the electrolytic cell is passed into the absorber tomake up for the spent caustic removed. It is preferred that the spentcaustic leaving the tower be largely converted to an alkali metalbisulfite through the absorption of S0 gas by the caustic solution,thereby preventing the absorption of substantial quantities of gasessuch as carbon dioxide 'which are less acidic than sulfur dioxide. It ispreferred that at least 50% of the available hydroxyl capacity for S0absorption in the caustic be utilized in the absorber in accordance withthe following reaction:

The solution of spent caustic (and any unreacted caustic) entering theneutralizer tank will mix with and become neutralized by the anolytesolution which is sufficiently acidic to stoichiometrically regeneratethe electrolyte, e.g. potassium sulfate. The acidic solution, initiallyobtained from the anode compartment of the electrolytic cell, isoptionally stored in an anolyte hold-up tank 33 and passed via line 34into the neutralizer tank by gravity or pumping means 35.

The mixture in the neutralizer may be stirred, if desired, by mixingmeans 36 with sufiicient space and time being allowed for disengagementof the liberated S0 The neutralization reactions occurring therein areas follows:

The overall result is the regeneration of the original electrolyticsolution accompanied by the release of a concentrated stream of gaseousS0 at exit line 37. The S0 gas collected can be purged from the stripperwith steam, vacuum or essentially the stoichiometric amount of air andultimately converted to sulfuric acid. The regenerated solution ofalkali metal sulfate or other salt is passed by a pump 38 throughparticle removing means such as a filter 39 prior to being returned as afeed solution to the electrolytic cell. Sedimentation, filtration,centrifugation, or other means of removing fly ash or particulate matterfrom the regenerated alkali metal sulfate solution prior to its recycleback to the cell is desirable to minimize plugging of the porousdiaphragm of the electrolytic cells.

The recovery of S0 by suitable contact of an S0 containing gas phasewith the aqueous caustic solution is both rapid and efiicient. The S0removal rate will of course depend on the size and type of the scrubber,the temperature, concentration and volume of the caustic employed, theabsolute pressure, flow rates of caustic and gas, S0 content of the gasphase, percent S0 removal, type of caustic employed (for example,whether potash or soda) concentration of other electrolytes in thecaustic, etc.

Analyzers may be employed to continuously record and monitor the S0level of the scrubbed gas effluent from the tower. The caustic solutioncan also be employed effectively to remove nitric oxide, nitrogendioxide, hydrogen sulfide, mercaptans and similar noxious gases that maybe present. It is understood that the invention disclosed herein may besimilarly applied to the removal of acidic sulfur contaminants of S0 S0H 8 and mercaptans from natural gas and petroleum fluids such asgasoline to produce a sweet product.

The process described in connection with FIG. 1 is particularlyeconomical for recovering sulfur dioxide from relatively small flows ofsulfurous gases, for example, from 1 to standard cubic feet per second.For such flows simplicity of operation is of great importance and thecost of scrubbing towers is relatively small. The process as describedsuffers from having a mass absorption coefficient in the tower (usuallyreferred to in chemical engineering treatises as K a) which is less thanthat obtainable with pure caustic owing to the presence in the causticof unconverted salt. The absorption tower must therefore be larger thanwould be required for pure caustic. On the other hand, the system isclosed, that is, it is not necessary to add or remove any substantialquantity of water or salt except to make up for any evaporation in theabsorption tower and the neutralizer. This simplicity is a distinctadvantage for small plants. However, for larger plants, generally thoseprocessing more than 100 standard cubic feet of gas per second, thereare economic advantages to producing pure caustic in the electrolyticcell. This results first in a substantial reduction in the size of theabsorption tower but also in the necessity of using an open cycleprocess in which water is added to the catholyte of the electrolyticcells and removed from the liquid effluent from the neutralizer,desorber, stripper or regenerator. This open cycle process will bedescribed by way of example by reference to the apparatus shownschematically in FIG. 2 and in particular to the employment of sodiumsulfate as the electrolyte feed solution to the electrolytic cell. InFIG. 2 equipment items which have the same function as those in FIG. 1are similarly numbered. In the process of FIG. 2, a feed solution ofsodium sulfate is passed from line 40 by pumping means (not shown) tothe electrolytic cell 1. By means of a source of direct current passedto the cell through leads 50 and 51 (source of current is not shown) thesodium sulfate is split into sodium hydroxide and a mixture of sulfuricacid and sodium sulfate referred to herein as sodium acid sulfate orsodium bisulfate. The electrolytic cell is preferably of the typedescribed in connection with FIG. 1 or FIG. 4. The catholyte compartment5, defined from the center compartment 3 by the cation membrane 6 isprovided with a caustic resistant cathode 11, such as copper, lead,nickel, nickel alloy, iron or steel and an inlet 12 through which wateris passed. The flow of water and electric current are regulated to givethe desired caustic concentration, preferably between 20 and 200 gramsof caustic per liter, which is withdrawn from the cell through theoutlet 13.

In operation a solution of sodium sulfate, preferably in the range ofabout 30 to 300 grams per liter, is introduced under pressure into thecenter compartment through inlet 10 at a rate and pressure which in itspassage through the porous diaphragm is sufiicient to prevent thehydrogen ions formed at the anode from migrating to the catholyte.Simultaneously water is passed into the cathode compartment via inlet 12at a rate depending upon the concentration of caustic desired in thecatholyte effiuent product and on the impressed direct electric current.Under the influence of the electric current, sodium ions pass throughthe cation selective membrane into the catholyte and combine withhydroxide ions produced by the electrolysis of water at the cathode. Thesolution in the center compartment 3, at least partially depleted ofsodium, passes through the porous diaphragm into the anode compartmentwhere combination of the sulfate anions with hydrogen ions produced bythe electrolysis of water at the anode forms the corresponding acid,that is, sodium acid sulfate and/or sulfuric acid. The anolyte iswithdrawn through outlet 8 as a mixture of unreacted salt, if any, andacid.

The effluent product from the cathode compartment is passed throughlfiie 15 into the optional caustic surge tank 16 and then through line18 by optional pumping means (not shown) or gravity into the absorber orscrub bing tower 19. A gas stream containing S is introduced into thetower at 20 through inlet 21. The gas after passing through the tower isremoved at exit line 28 at least partially depleted in sulfur dioxide.It is preferred to remove from 80 to 99 percent of the sulfur dioxidecontent.

The rich electrolyte containing absorbed sulfur dioxide in the form ofsodium sulfite and/or bisulfite is carried from the tower through outlet31 and passed into the neutralizer (desorber, stripper or regeneration)tank 32 and there mixed with and neutralized by the acidic solutionobtained from the anode compartment of the electrolytic cell. Theanolyte is stored in anolyte surge tank 33 and passed through line 34into the stripper tank 32. The result is the regeneration of the sodiumsulfate accompanied by the release of a concentrated, humid stream ofgaseous sulfur dioxide through exit 37. The sodium sulfate leaves thestripper through line 61 and is preferably stripped substantially freeof sulfur dioxide by injecting live steam through inlet and sparger 60.

It will be clear that all of the water which is introduced into thecatholyte at 12 and the sparger at 60 occurs in the efiluent sodiumsulfate solution except for that lost in the gas efiluent lines 28 and37. This water must be removed, for example, by evaporator 62 before thesodium sulfate is recycled to the electrolysis cell. The evaporator ispreferably of the multiple effect, fiash or vapor recompression type.Steam or other heat exchange media is introduced at inlet 63 and issuesat exit 64. Water vapor or liquid water substantially free of sodiumsulfate issues at 65. Alternatively, the sodium sulfate may beconcentrated by employing reverse osmosis or electrodialysis processes.The regenerated sodium sulfate solution is passed through clarificationmeans 39 prior to passage as feed solution to the electrolytic cell.

A preferred method of concentrating the sodium sulfate efiluent from thestripper 32 is shown schematically in FIG. 3 in which equipment itemshave the same function as those similarly numbered in FIGS. 1 and 2. Therich electrolyte contianing absorbed sulfur dioxide in the form ofsodium sulfite and/or bisulfite is carried from the absorption tower 19through outlet 31 and passed into the neutralizer or stripper tank 32and there mixed with and neutralized by the acidic solution obtainedfrom the anode compartment of the electrolytic cell. The result is theregeneration of the sodium sulfate accompanied by the release of aconcentrated, humid stream of gaseous sulfur dioxide through exit 37.The sodium sulfate solution is less concentrated than that fed into thecentral compartment 3 of the electrolytic cell since it comprises, inaddition, substantially that water introduced into the catholytecompartment through entrance means 12. The sodium sulfate leaves thestripper tank 32 through line 61 and is preferably strippedsubstantially free of sulfur dioxide by injecting live steam throughinlet and sparger 60. The excess water is removed by leading theeflluent dilute sodium sulfate solution through line 61 to thehumidifying chamber 62 wherein it contacts the influent sulfurous gasstream entering at 21. Generally, the latter is at an elevatedtemperature, for example, 300 to 400 F., and is not saturated with watervapor. In the humidifying chamber 62, therefore, part of the water inthe dilute sodium sulfate solution will be evaporated and a suitablyconcentrated solution can be withdrawn through line 66 and, afterremoval of solid matter in clarifier 39, can be recycled to theelectrolytic cell. Simultaneously, the sulfurous gas stream will bepartially cooled and humidified and will pass to the absorption tower 19for recovery of sulfur dioxide. For many sulfurous gases, the excesswater content of the dilute sodium sulfate solution will not besufiicient to humidify the gas stream and if this is the case theneither part of the gas stream must be by-passed around the humidifyingchamber to the absorber or additional water or steam must be injectedthrough line 67. If the liquid in the absorption tower 19 iscomparatively dilute, then it is preferred to by-pass part of the gasstream around the humidifying chamber. If the liquid in tower 19 iscomparatively concentrated, then it is preferred to inject extra wateror steam into the humidifying chamber.

The invention has been described schematically in connection with asingle three-compartment electrolytic cell. It will be understood thatfor practical applications, a multiplicity of such cells will berequired. A particularly advantageous multicell configuration is shownin FIG. 4 utilizing bimetallic, bipolar electrodes in which 7 areanodes, for example, of antimony-lead, silver-lead, calcium-lead,chillex, a tungsten bronze, platinum or platinum-plated titanium,zirconium, niobium or tantalum. The anodes preferably have a thicknessin the range of 0.01 to 0.3 centimeter.

The microporous diaphragms 4 have a void volume of at least 50 percent,a thickness in the range of 0.1 to 1.0 millimeter and an average poresize in the range of 10 to microns.

Preferred materials of construction for diaphragms are rubber, includingsynthetic rubber, ceramics, polyethylene, polypropylene,ethylene-propylene copolymers and terpolymers, polyvinyl chloride,polyvinyl acetate, copolymers of vinyl acetate and vinyl chloride,copolymers of ethylene and vinyl acetate, polyvinylidene chloride,copolymers of vinylidene chloride and vinyl chloride, polyacrylonitrile,copolymers of acrylonitrile and vinyl chloride, nylon, wool, copolymersof styrene and butadiene, cellulose, regenerated cellulose, celluloseacetate, burlap, canvas, asbestos, polytetrafiuoroethylene,polyvinylidene fluoride, polyvinyl fluoride,polychlorotrifluoroethylene, epoxy-bonded glass fiber mats, polyesterbonded glass-fiber mats, polystyrene bonded glass fibers and the like.The cation selective membranes 6 have a water content in the range of 10to 40 percent of the dry weight, a cation exchange capacity of 1 to 10milliequivalents per gram of dry weight, a thickness in the range of 0.1to 1.0 milllimeter, pore sizes of less than 0.1 micron, an arealresistance in equilibrium with 1 molar sodium hydroxide of not more than10 ohm cm? at the operating temperature of the cell, a transport numberfor sodium ion of at least 0.5 when in equilibrium with 1 molar sodiumhydroxide, a Mullen A burst strength of at least 30 pounds per squareinch. Suitable materials are crosslinked polystyrene sulfonate salt,crosslinked polyethyl styrene sulfonate salt, crosslinked copolymers ofethyl styrene sulfonate salt and styrene sulfonate salt, sulfonatedcrosslinked polystyrene, sulfonated crosslinked polyethylstyrene,sulfonated crosslinked copolymers of styrene and ethyl styrene,sulfonated crosslinked polymers of vinyl toluene, sulfonated crosslinkedcopolymers of vinyl toluene and ethyl styrene, crosslinked polyacrylatesalts, crosslinked polymethacrylate salts, crosslinked copolymers ofacrylate and methacrylate salts, crosslinked copolymers of styrene andmaleate or fumarate salts, also, the phosphates, arsenates, molybdates,vanadates, niobates, chromates, manganates, tantalates and/or tungstatesof titanium, zirconium, haf- 9 niurn, tin, thorium, lead and/or cerium.The inorganic materials are bonded vvtih film-forming organic andinorganic materials. Other cation exchanging substances may also beused.

The cathodes 1 1 preferably have a thickness in the range of 0.01 to0.30 centimeter and consist of lead, antimony-lead, silver-lead,calcium-lead, chilex, a tungsten bronze, copper, nickel, nickel alloys,cadmium, tin, Monel, bronze, brass, aluminum, silver, graphite or gold,platinum or palladium plated titanium, zirconium, or niobium.

The interior electrodes, such as 52, are preferably bipolar and areadvantageously bimetallic. Thus the interior electrodes may for examplebe titanium sheets each face of which is coated with from 0.25 to 2.5micron of platinum or other nobel metal. Alternatively, the electrodesmay be a tungsten bronze or a lead alloy. Preferred bimetallic, bipolarinterior electrodes include those in Which the cathode surface 11 isnickel and the anode surface 7 is platinum or platinum-plated tantalumor niobium. It has been found that for best results the distance (A)between the cathode surface 11 and the adjacent surface of the cationselective membrane 6 should be in the range of 0.5 to 5 millimeters andthe current density should be in the range of 50 to 250 milliamperes persquare centimeter. If the current density is less than 50 milliamperesper square centimeter, then the flow rates required to achievesatisfactory concentra tions of caustic will be so low that some regionsof the catholyte will be comparatively stagnant and the currentefficiency will decrease, apparently because the caustic concentrationhas been excessive in such regions. On the other hand, if the currentdensity is in excess of 250 milliamperes per square centimeter, then itis found that the useful life of the cation selective membranes decreasesubstantially, possibly due to excessive heat generation in the bulk ofthe membrane, perhaps coupled with non-uniform current distributioncaused by gasbinding. If the cathodemembrane spacing (A) is less than0.5 millimeter, then gas-binding interferes substantially with the flowof electric current and may result in rapid fluctuation of the current.If the cathode-membrane spacing (A) is greater than 5 millimeters, thenthe flow rates to achieve satisfactory concentrations of caustic willresult in linear velocities which are so low that some regions of thecatholyte will be comparatively stagnant with the results describedabove. It has been found that if the membrane 6 is less than 0.1millimeter thick, it will have a tendency to bow either to- Ward or awayfrom the cathode in either case resulting in non-uniform flow in thecathode, stagnation, gas-binding and non-uniform distribution of thecurrent. On the other hand, if the membrane is thicker than 1.0millimeter, then the useful lifetime of the membrane is reducedapparently because it is diificult to remove from the interior of themembrane the heat generated at the high current densities employed. Ifthe pore sizes in the cation selective membrane 6 are in excess of 0.1micron, then hydraulic flow through the membranes becomes excessive,caustic can be lost from the catholyte into the central compartment 3and from the latter into the anolyte resulting in a loss of currentefficiency. Further control of the flow of electrolyte from the centralcompartment 3 into the anolyte through the porous diaphragm is verydifficult when the average pore size of the cation selective membrane isgreater than 0.1 micron. At high pressures, part of the electrolyte willflow through the cation selective membrane into the catholyte. At lowpressures, part of the catholyte may flow through the cation selectivemembrane into the central compartment resulting in a loss of currentefliciency. The spacing (B) between the anode 7 and the adjacentdiaphragm surface 4 should be in the range of 0.5 to 5.0 millimeters forthe reasons discussed in connection with the cathode-cation membranespacing. If the diaphragm is thicker than 1.0 millimeter, then thepressure required to force the electrolyte through the diaphragm willresult in excessive bowing of the cation selective membrane with theeffects on catholyte stagnation and current distribution discussedabove. If the diaphragm is less than 0.1 millimeter thick, it will tendto bow either toward or away from the anode. In either case resulting innon-uniform anolyte and current distribution, in reduced currentefficiency and in decreased anode and diaphragm life. If the electrolytein the central compartment 3 has a concentration of less than 30 gramsper liter, the heat generation in the centralcompartment and in theanolyte will be excessive and result in reduced life of the diaphragmand the anode. It appears that the deterioration of the anode atconcentrations below 30 grams per liter and at the high currentdensities required is not solely due to heat effects but may be due toother causes not well understood. If the electrolyte in the centralcompartment has a concentration greater than 300 grams per liter, atpractical conversions the current efficiency in the catholyte will begreatly reduced. Similarly, it is found that the caustic in the cathodecompartment 5 should have a concentration in the range of 0.5 to 5.0equivalents per liter. If the concentration is less than 0.5 normal,heat generation is excessive and the life of the membrane is reduced. Ifthe concentration is greater than 5 .0 normal, the life of the membraneis also reduced in this case apparently owing to some sort of chemicalattack and the current efiiciency of the anolyte is reduced drastically.

The following examples show by further illustration and not by way oflimitation the cyclic method of absorbing S0 and the regeneration of thespent liquid absorbent.

EXAMPLE I An array of three electrolytic cells of the general typedisclosed and described in connection with FIG. 4, containing 3platinum-coated titanium anodes and 3 nickel cathodes is used to converta 2 normal aqueous solution of sodium sulfate into sodium acid sulfateinto sodium acid sulfate and sodium hydroxide. The diaphragms aremicroporous silcone rubber and have a thickness of 0.25 millimeter andthe supported on their anode sides by platinum-plated expanded titaniumsheet having an expanded thickness of 2 millimeters which thusdetermines the diaphragm-anode spacing. The void volume of the diaphragmis about 70 percent and the average pore size is about 20 microns. Theinterior electrodes are bimetallic and bipolar, that is, they consist ofa laminate of titanium and nickel. The active surfaces of all theelectrodes are scribed to increase the effective surface area and theplatinum is plated on the titanium after scribing. The platinum plate isabout 50 microinches (1.25 microns) thick. The membrane isself-supporting carboxylic type cation permselective membrane of thetype described in US. Pat. No. 2,731,408, prepared from a mixturedivinyl benzene, ethyl styrene and acrylic acid. It has a thickness of0.7 millimeter, an areal resistance of 2 ohm cm. in 1 molar sodiumhydroxide at 150 F., a water content of about 20 percent of its dryweight, a cation exchange capacity of about 6.5 milliequlvalents per drygram of resin, average pore sizes of less than 0.1 micron, a transportnumber for sodium ions of about 0.85 when in equillibrium with 1 molarsodium hydroxide, a Mullen A burst strength of about pounds per squareinch and is reinforced with two layers of bonded, non-wovenpolypropylene mat. The membranes are supported on their cathode sides byexpanded nickel sheet having an expanded thickness of 2 millimeterswhich thus deter mines the membrane-cathode spacing. The spacing betweenthe diaphragm and the membrane is filled with non-woven bondedpolypropylene screen having a thickness of 2 millimeters. The outeredges of the compartments are fitted with high density polyethylenegaskets having a compressed thickness of about 2 millimeters. The sodiumsulfate solution is introduced into the central compartments at a rateof 4 liters per hour per active square foot of anode. The currentdensity at the anode and at the cathode is 120 amperes per square foot.The temperature of the cell is maintained at 150 F. by recirculatingboth the anolyte and the catholyte through heat exchangers. The voltagerequired is about 15 volts D.C. that is, about volts per cell. At steadystate the bleed from the anolyte is found to be essentially sodiumbisulfate indicating a current efficiency of about 90 percent. At thecathode, 4 liters of caustic per hour per square foot are removed fromthe recirculating catholyte stream and the volume is maintained byadding fresh water. At steady state the catholyte bleed is found to havea concentration of about 1 equivalent per liter indicating a currentefliciency of about 90 percent. The catholyte bleed is contactedcounter-currently with a simulated flue gas stream having the followingcomposition:

Component: Volume percent S0 0.3 CO 13.0 N 74.0 0 6.0 N 0 6.7

The contact is carried out in a column packed with glass Raschig rings.The liquid and the gas flows and the height of the packing are adjustedto remove about 90 percent of the S0 and give a liquid effluent havingan empirical composition corresponding to about 82 mol percent of sodiumbisulfite and about 18 mol percent of sodium sulfite. The liquideffluent is mixed with the corresponding amount of anolyte from theelectrolytic cell and passed downwardly through a second column packedwith glass Raschig rings against an upward stream of air adjusted togive a gaseous efiiuent having the following range of analyses on a drybasis:

Component: Volume percent S0 25 to 28 0 19 to 12 N 56 to 50 and thussuitable for the manufacture of sulfuric acid using the contact process.Alternatively, the sulfur dioxide may be stripped by injecting steam inthe bottom of the packed tower or reducing the pressure in the towerwith a mechanical vacuum pump. The sodium sulfate leaving the bottom ofthe column is concentrated to about 2 equivalents per liter in amultiple effect evaporator. The condensate is used as feed to thecatholytes of the multiple electrolytic cell. The sodium sulfateeffluent is used as feed to the central compartments of the cell therebycompleting the cyclic operation.

EXAMPLE 2.

This examle simulates the recovery of sulfur dioxide from a hot stackgas using potassium sulfate. The synthetic flue gas of Example 1 isheated to 325 F. in an electrically heated Alundum tube and contactedcountercurrently in a third column packed with glass Raschig ringsagainst the downwardly flowing dilute potassium sulfate solution issuingfrom the second (stripping) column. It is found that to maintain aconcentration of 2 equivalents per liter in the liquid efiluent it isnecessary at steady state either to add some water to the dilutepotassium sulfate influent to the column or to the concentrated efiiuentfrom the column. Alternatively, part of the sulfurous gas may beby-passed around the tower. Fly ash recovered from a Cotrellprecipitator is ground to pass 325 mesh and added with stirring to theresulting potassium sulfate solution at the rate of 200 milligrams perliter to simulate the pickup of fly ash which would occur from thegaseous effluent from a power house. The resulting mixture is allowed tosettle and then filtered through diatomaceous earth. The filtrate issent to the control compartments of the multiple elecrolytic cellapparatus of Example 1 which uses silverlead anodes and copper cathodes.The cation selective membrane is polyvinylidene fluoride bondedzirconium phosphate prepared according to the method of United StatesDepartment of the Interior Oflice of Saline Water Research andDevelopment Progress Report No. 148 and has a thickness of about 0.3millimeter, a cation exchange capacity of about 2.5 milliequivalents perdry gram, an areal resistance of 2 ohms cm? in 1 molar sodium hydroxideat 170 F., a water content of about 12 percent of the dry weight,average pore sizes of less than 0.1 micron, a transport number forsodium ions of about 0.85 when in equilibrium with 1 molar sodiumhydroxide, a Mullen A burst strength of about pounds per square inch.The membranes are supported on their cathode sides by a perforated,corrugated sheet of polyvinyl chloride having a thickness of about 2millimeters. The diaphragms are asbestos paper having a thickness ofabout 0.2 millimeter and are supported on their anode sides by theperforated corrugated polyvinyl chloride sheet referred to above. Thevoid volume of the diaphragms is about percent and the average pore sizeis about 30 microns. The interior electrodes are bimetallic and bipolar,that is, they consist of a laminate of silver-lead and copper. Thespacing between the diaphragm and the membrane is filled with thenon-woven bonded polypropylene screen used in Example 1. The gaskets atthe edges of the compartments are butyl rubber having a compressedthickness of about 2 millimeters. The potassium sulfate solution isintroduced into the central compartments at a rate of 6 liters per hourper active square foot of anode. The current density at the anode and atthe cathode is 180 amperes per square foot. The temperature of the cellis maintained at 170 F. by recirculating both the anolyte and thecatholyte through heat exchanges. The voltage required is about 23 voltsD.C., that is, about 8 volts per cell. At steady state the bleed fromthe anolyte is found to be essentially potassium bisulfate indicating acurrent efficiency of about percent. At the cathode, 6 liters of causticper hour per square foot are removed from the recirculating catholytestream and fresh water is added continuously to maintain the in-processvolume. At steady state the catholyte bleed is found to have aconcentration of about 1 equivalent per liter indicating a currentefiiciency of about 90 percent. The catholyte bleed is contactedcountercurrently with the humidified and partially cooled, simulatedflue gas stream from the sulfate concentration (third) column. Thecontact is carried out in a (first) column packed with ceramic Berlsaddles. The liquid and the gas flows and the height of the packing areadjusted to remove about percent of the S0 and give a liquid effluenthaving an empirical composition corresponding to about 67 mol percent ofpotassium bisulfite and 33 mol percent of potassium sulfite. The liquidefiluent is mixed with the corresponding amount of anolyte from theelectrolytic cell and passed downwardly through the second column. Livesteam is injected in the bottom of the column to raise the temperatureof the liquid effluent to 200 F. The gaseous eflluent from the tower iscooled in a partial condenser, the liquid flowing back to the strippingtower. The partially dried sulfur dioxide is dried over silica gel andthe S0 liquified at 40 F. The potassium sulfate leaving the bottom ofthe column is sent to the humidifying column thus completing the cycle.

In other experiments in which the void volume of the diaphragm is lessthan 50 percent, the thickness is not in the range of 0.1 to 1.0millimeter or the average pore size is not in the range of 10 tomicrons, it is found that continuous, stable, cyclic operation cannot beachieved. If the void volume is less than 50 percent, the thickness isgreater than 1.0 millimeter or less than 0.1 millimeter, or the averagepore size is less than 10 microns or greater than 100 microns, thecurrent efliciency in the catholyte is substantially less than 100percent and the life of the cation selective membrane is seriouslydiminished. In another series of experiments, it is found that if thecation selective membrane has a capacity of less than 1 millequivalentper dry gram, a thickness of less than 0.1 millimeter, a water contentof more than 40 percent, pore sizes greater than 0.1 micron or atransport number for sodium ions which is less than 0.5 when inequilibrium with 1 molar sodium hydroxide, the current efliciency in theanolyte is substantially less than 100 percent. It is also found that ifthe membrane has a water content of less than 10 percent of the dryweight, a thickness greater than 1.0 millimeter, an areal resistance inequilibrium with 1 molar sodium hydroxide of more than 10 ohm cm. at theoperating temperature of the cell or a Mullen A burst strength of lessthan 30 pounds per square inch, the useful life of the membrane isreduced to impractical values at practical current densities.

We claim:

1. A cyclic system for the continuous removal of sulfur dioxide fromsulfur dioxide containing gases comprising:

(a) an eletrolytic cell for converting aqueous alkaline metal salts intotheir corresponding acid and caustic alkaline solutions;

(b) liquide efliuent means on cell for separately removing said acid andcaustic solutions from the anode and cathode areas, respectively, ofsaid cell;

(c) communicating means between said liquid effluent means of saidcathode area to scrubber means for intimately contacting said Scontaining gases with said caustic solution causing a major proportionof said caustic solution to be converted to bisulfite;

(d) combining means commuicating with said scrubber means for combiningthe resulting-caustic bisulfite solution with said removed acid solutionresulting in a neutralization reaction and the desorption and release ofsulfur dioxide gas therefrom;

(e) recover means communicating with said combining means for recoveringsaid released sulfur dioxide gas; and

(f) return means communicating with said combining means for returningat least a portion of the neutralized solution as a feed solution backto said electrolytic cell, thus completing the cyclic system.

2. The system of claim 1 characterized in that the desorbtion andrelease of sulfur dioxide gas is eiiected by reboiling meanscommunicating with said combining means for reboiling said neutralizedsolution.

3. The cylic system of claim 1 wherein the electrolytic cell consists ofthree compartments, the cathode compartment of which is separated fromthe center compartment by a cation-selective ion exchange membrane, aspaced fluid-permeable diaphragm separating said center compartment fromthe anode compartment, means for passing the aqueous feed solution to'be converted into at least the center compartment of said electrolyticcell and means for maintaining a greater pressure in said centercompartment than the pressure in said anode compartment to cause saidfeed solution to flow from the center compartment through the porousdiaphragm into and out of said anode compartment.

4. The cyclic system of claim 3 which is provided with a multiplicity ofsaid electrolytic cells containing bimetallic, bipolar, interiorelectrodes.

5. The system of claim 1 wherein recycling means communicating with saidscrubber means is provided,

for repeated contact of said caustic solution with the sulfur dioxidecontaining gas.

6. The cyclic system of claim 1 wherein concentrating means is providedfor concentrating said neutralized removed solution as the returned feedsolution to said electrolytic cell.

7. An apparatus for the recovering of sulfur dioxide from asubstantially arid gas containing the same which 1 comprises:

(a) liquid-gas contact means for intimately contacting said arid gaswith an aqueous alkali metal sulfate salt solution to concentrate saidsalt solution and humidity said arid gas;

(b) influeut means communicating with said liquidgas contact means forfeeding under pressure at least a portion of said concentrated sulfatesalt solution into the central compartment of a three compartmentelectrolytic cell containing an anode and cathode compartment, thecentral compartment being separated from the anode compartment by aporous diaphragm thus casuing said salt solution to pass through theporous diaphragm into the anode compartment;

(c) liquid effiuent means on said cell for removing acidic and causticsolutions from the anode and the cathode compartments respectively;

(d) means connected to the electrodes of said cell for passing a directelectric current through said cell;

(e) communicating means between said liquid eflluent means of saidcathode compartment to scrubber means for intimately contacting theeffiuent caustic solution from the cathode compartment with saidpartially humidified arid gas to convert at least a portion of saidcaustic solution into alkali metal bisulfite, with combining meanscommunicating with scrubber means for combining the saidcaustic-bisulfite solution with said removed acidic solution, resultingin a reforming of alkali metal sulfate solution;

(f) stripping means in communication with said combining means for atleast partially stripping sulfur dioxide from said reformed alkali metalsulfate salt solution; and

(g) recovery means communicating with said combining means forrecovering said sulfur dioxide as a by-product.

8. The apparatus of claim 7 characterized in that reboiling means incommunication with said combining means for reboiling said reformedalkali metal sulfate salt solution is employed for stripping said sulfurdioxide.

References Cited UNITED STATES PATENTS 2,768,945 10/ 1956 Shapiro 23-2X3,344,050 9/1967 Mayland et al 234X 3,479,261 1 1/ 1969 Heredy 204-61HOWARD S. WILLIAMS, Primary Examiner A. C. PRESCOTT, Assistant ExaminerUS. 01. X.R. 204-

